The present invention generally relates to ultrasound imaging. In particular, the present invention relates to real-time motion correction for ultrasound spatial compound imaging.
Ultrasound is sound having a frequency that is higher than a normal person may hear. Ultrasound imaging utilizes ultrasound waves or vibrations in the frequency spectrum above normal human hearing, such as the 2.5-10 MHz range. Ultrasound imaging systems transmit ultrasound into a subject, such as a patient, in short bursts. Echoes are reflected back to the system from the subject. Diagnostic images may be produced from the echoes. Ultrasound imaging techniques are similar to those used in sonar and radar.
A medical ultrasound system forms an image by sequentially acquiring echo signals from ultrasound beams transmitted to an object being imaged. An individual beam is formed by transmitting a focused pulse and receiving the echoes over a continuous range of depths. An amplitude of an echo signal decreases significantly for signal reflectors located deeper in the object due to increased signal attenuation of intervening structures, such as intervening tissue layers. Therefore, a signal-to-noise ratio decreases since noise generated by the ultrasound system's signal amplifiers, for example, may not be reduced to arbitrary low levels.
Forming the best possible image at all times for different anatomies and patient types is important to diagnostic imaging systems. Poor image quality may prevent reliable analysis of the image. For example, a decrease in image contrast quality may yield an unreliable image that is not usable clinically. Additionally, the advent of real-time imaging systems has increased the importance of generating clear, high quality images.
B-Mode or “Brightness” mode is a common display format for an ultrasound image. Currently, B-Mode ultrasound imaging system transducers fire a narrow ultrasound beam or vector in a single direction. The transducer array then waits to listen to all echoes returning from reflectors along that same straight line. Strength of the return echoes is used to represent a reflectivity of an object. Reflectivity of an object, such as an anatomy of a patient, is typically calculated using a range equation. The range equation determines that time equals signal round trip divided by speed of sound in a subject medium. Current ultrasound systems utilize receive beamforming to reconstruct an object being imaged. That is, an ultrasound system listens for echo signals using a receiver which is then used for beam reconstruction and beam directional determination. The scheme of firing an ultrasound beam and listening for reflected echoes is repeated sequentially in a number of directions to span a two-dimensional section in an object space, such as an anatomical space. The ultrasound system paints each line that is determined from the reflected echo signals with a brightness level corresponding to a return echo signal strength. A complete set of vectors that sweep out a section of space constitutes a B-Mode frame. The reconstruction process is repeated to successively paint frames and achieve a standard real-time B-Mode display.
Spatial compounding has become an advanced and important diagnostic tool in a wide range of applications in ultrasound imaging. In spatial compounding, a target is scanned from several angles of insonification or irradiation with sound or other such waves. Multiple received images are then combined or averaged to form a single image. A compounded image typically shows less speckle or interference introduced by scattering which degrades image resolution. A compounded image may also provide better specular reflector delineation than conventional ultrasound images from a single angle. In some ultrasound machines, multi-angle spatial compounding has been implemented on different types of transducers, such as a one-dimensional linear array and a one-dimensional curved linear array.
Spatial compound imaging improves contrast resolution by reducing speckles and enhancing tissue boundaries. However, a spatial compounded image is a summation of a series of frames, which are collected at different times. Consequently, spatial compounding is susceptible to motion, such as tissue or probe motion. Additionally, imaging with an extended field of view results in blurriness or artifacts from regular motion. Therefore, an improved method and apparatus for reducing or eliminating motion effects, such as blurriness, in a spatial compounded image would be highly desirable.